This invention relates to an apparatus and method for detecting defects in a substrate and, in particular, to the detection of defects located along the peripheral edge of the substrate by scanning with an image pickup device.
Semiconductor wafers and other substrates are subject to breakage, cracks and discoloration which can make them unsuitable for their intended use. A known method of testing substrates for such imperfections is to raster-scan the substrate with an image pickup device such as an industrial television camera. In this method, the image of the substrate on the photoelectric screen of the camera tube is scanned horizontally by an electron beam, line by line, until the entire substrate has been viewed. When so scanned, an electric current flows at the output of the pickup device which has an instantaneous magnitude proportional to the brightness of the portion of the image scanned. Defects are detected by observing a change in the output level of the pickup device as the substrate is scanned.
When a defect is located toward the center of the substrate, that is, away from its periphery, the change in the output level of the pickup device is relatively easy to detect. However, if the defect is at or close to the periphery or contour of the substrate, it can be quite difficult to distinguish the peripherial defect from the surrounding material by observing only the change in output level during horizontal scanning. Nevertheless, it is just as important to find peripheral defects as those located relatively far from the edge of the substrate.
A method for detecting a defect at the periphery of a circular sample is disclosed in a Japanese Patent Application No. 52-83745.
In accordance with this method, the coordinates of a standard substrate are stored in a memory and the coordinates of the sample to be tested for defects are conformed with those of the standard on a point-by-point basis. Any difference between the coordinates measured for the sample and the coordinates of the standard indicates that a defect is present in the sample. With this method, a large change in the contour can easily be detected but a small change is hard to detect. Consequently, the method is not suitable for the detection of small defects in the peripheral region of the sample and the method is influenced by a change in the size of the sample. Also, since the coordinates are compared one after the other, processing time is lengthy, and a memory is needed to store the coordinates which has the disadvantage of increased cost.
Accordingly, it is an object of the present invention to overcome the disadvantages of the prior art by providing a low-cost defect detecting apparatus and method which permits location of defects forming part of the contour or periphery of a substrate as well as defects located in the inner portion of the substrate. Other objects are to reduce the processing time so that the detection is completed simultaneously with the completion of the image pickup of one picture frame and eliminate the need for a memory to store coordinates.